This invention relates to an adjustable cyclically operated switching arrangement and more particularly is directed to such a switching arrangement for variably controlling the average microwave power in a microwave oven.
A conventional technique for reducing the average power of the microwaves generated by a magnetron and delivered, for example, to the cooking cavity of a microwave oven or the like, has been to energize the magnetron through a coupling transformer in a periodic or cyclical manner. In this way within a given, say 5 minutes, operational duration, cyclical energization and de-energization of the magnetron such that the energized time equals the de-energized time will reduce the effective average power of the microwaves in the cooking cavity to an average of 50% of the power that would be delivered if the magnetron were energized continuously over that five minute duration.
The capability of reducing the average power of the microwaves in a microwave oven is an important feature when using the microwave oven to expedite the defrosting of frozen food, as is clearly described in U.S. Pat. No. 3,842,233, issued Oct. 19, 1974, which patent is assigned to the same assignee as the instant application. In the mentioned patent a current limiting resistor is connected in series circit relation with the primary winding of the coupling transformer for a short period of time at the beginning of each cyclical energization of the magnetron in order to limit current surges in the circuit, which surges might otherwise drive the transformer to saturation, reduce the effective life of the magnetron, and damage switches or other components of the circuit. After the short interval at the beginning of each energization cycle of the magnetron, the current limiting resistor is effectively short circuited or bypassed in order to provide full power to the coupling transformer and magnetron.
One drawback to the cyclical operation of the magnetron to reduce the effective power in a microwave oven for food defrosting purposes is that the cycling frequency is usually a fixed frequency and the duration within each cycle that the magnetron is energized is also fixed. Therefore, such relatively fixed defrost mode circuits are effective only for defrosting efficiently certain types of foods, but may not be so efficient for defrosting other types of foods or for providing an adjusting cooking rate for already defrosted foods.